Starlink Project
Starlink Cookbook 7.2

Martin Clayton and Anthony Holloway

15 June 1998

Simple Spectroscopy Reductions


An extracted spectrum and its corresponding arc.
Graphic created using DIPSO[11].


This Cookbook describes the basic concepts and methods used in optical astronomical spectroscopy; it is aimed at those new to the field. Complete worked example reductions for both one- and two-dimensional longslit spectra, using real datasets, are described. Common problems and their solutions are discussed. A section on related resources is included, as is a glossary of commonly used terms.


1 Introduction
 1.1 How to Use this Cookbook
 1.2 Other Sources of Information
2 Simple 1-D and 2-D Spectra
3 Preparing for Observation
 3.1 What CCD Data Do You Need?
 3.2 Detector Pre-processing
 3.3 Flat Fields
 3.4 1-D Spectrum Tracing or 2-D Frame De-rotation
 3.5 Wavelength Calibration
 3.6 Flux Calibration
4 Basic Steps in 1-D Data Reduction
 4.1 Image Preparation
  4.1.1 Software for CCD Data Preparation
 4.2 Finding the Spectrum in the Image
 4.3 Choosing the Object and Background Channels
 4.4 Tracing the Spectrum
 4.5 Flat Fielding
 4.6 Extracting the Spectrum
 4.7 Wavelength Calibration
 4.8 Finishing Reduction
  4.8.1 Flux Calibration & Extinction Correction
  4.8.2 Normalisation
5 A Worked Example
 5.1 Setting Up
 5.2 Inspecting the Images
 5.3 Locate the Spectrum
 5.4 Tracing the Spectrum
 5.5 Choosing the Object and Background Channels
 5.6 Flat Fielding
 5.7 Extracting the Spectrum
 5.8 Looking at the Extracted Spectrum
 5.9 Wavelength Calibration
 5.10 Normalising the Spectrum
 5.11 Output the Spectrum
6 Longslit (2-D) Data Reduction
 6.1 Image Preparation
 6.2 Wavelength and curvature calibration
 6.3 Scrunching
 6.4 Flux calibration
7 A 2-D Worked Example
 7.1 Setting Up
 7.2 Inspecting the 2-D images
 7.3 Arc line fitting
 7.4 Arc line identification
 7.5 Curvature correction
 7.6 Calibrating the wavelength axis
 7.7 Scrunching
 7.8 Blaze correction
8 Cookies
 8.1 How to Use the echmenu Parameter Editor
 8.2 Looking at FITS Header Cards: Air Mass
 8.3 Looking at FITS Header Cards: Wavelength Coverage
 8.4 Looking at FITS Header Cards: CCD Characteristics
 8.5 Problems Finding the Spectrum
 8.6 Tracing Strategies
 8.7 What if the Slit is More Than 21-Pixels Wide?
 8.8 Better Flat-Field Models
 8.9 Reviewing the Flat Field
 8.10 I Can’t Recognise the Arc Spectrum
 8.11 Flux Calibration
9 Glossary


[1]   L. Achmad and L. Pasquini,
CASPEC Thorium-Argon Atlas in the 3050–3650Å Region,
ESO Document ASD-MAN-SCI-000-001, May 1995.

[2]   M. J. Bly & P. M. Allen,
XDISPLAY Setting Remote Xwindows,
Starlink User Note 129.4, March 1996.

[3]   S. D’Odorico, M. Ghigo and D. Ponz,
An Atlas of the Thorium-Argon Spectrum for the ESO Echelle Spectrograph in the λλ 3400–9000Å Region,
ESO, 1987.

[4]   Michael Bessell and Max Pettini,
UCLES Spectrum of the Thorium-Argon Hollow-Cathode Lamp,
AAO UM 28.1, January 1991.

[5]   Martin Clayton,
Introduction to Echelle Spectroscopy,
Starlink Guide 9.2, October 1996.

[6]   Martin Clayton,
Echelle Data Reduction Cookbook,
Starlink Cookbook 3.1, August 1996.

[7]   Malcolm J. Currie, G. J. Privett & A. J. Chipperfield,
CONVERT, A Format-conversion Package,
Starlink User Note 55.6, December 1995.

[8]   Malcolm J. Currie,
KAPPA—Kernel Application Package,
Starlink User Note 95.8, August 1992.

[9]   Peter W. Draper,
CCDPACK—CCD Data Reduction Package,
Starlink User Note 139.4, November 1995.

[10]   Keith Horne,
An Optimal Extraction Algorithm for CCD Spectroscopy,
pp. 609–617, Pub. A. S. P. Vol. 98, June 1986.

[11]   I. D. Howarth, J. Murray, D. Mills & D. S. Berry,
DIPSO (V3.3)—A Friendly Spectrum Analysis Program,
Starlink User Note 50.19, January 1996.

[12]   Image Processing Group, ESO,
MIDAS Users Guide,
Volumes B and C, November 1994.

[13]   Tom Marsh,
The Extraction of Highly Distorted Spectra,
pp. 1032–1037, Pub. A. S. P. Vol. 101, November 1989.

[14]   Philip Massey,
A User’s Guide to CCD Reductions with IRAF,
Central Computer Services, NOAO, June 1992.

[15]   Ian S. McLean,
Electronic and Computer-Aided Astronomy,
Ellis Horwood Ltd., 1989.

[16]   Dave Mills, John Webb and Martin Clayton,
ECHOMOP—Echelle Data Reduction Package,
Starlink User Note 152.2, December 1995.

[17]   J. G. Robertson,
Optimal Extraction of Single-Object Spectra from Observations with Two-Dimensional Detectors,
pp. 1220–1231, Pub. A. S. P. Vol. 98, November 1986.

[18]   Keith Shortridge, Horst Meyerdierks, Malcolm Currie and Martin Clayton,
FIGARO—A General Data Reduction System,
Starlink User Note 86.12, October 1996.

[19]   Francisco Valdes,
Guide to the Slit Spectra Reduction Task DOSLIT,
Central Computer Services, NOAO, April 1992.

[20]   Francisco Valdes,
IRAF Tutorials: Extraction of Stellar Long Slit Spectra using DOSLIT,,
April 1994.

[21]   R. F. Warren-Smith & M. .D. Lawden,
HDS, Hierarchical Data System,
Starlink User Note 92.10, July 1995.

[22]   R. F. Warren-Smith,
NDF, Routines for Accessing the Extensible N-Dimensional Data Format,
Starlink User Note 33.4, March 1995.

[23]   T. N. Wilkins, D. J. Axon, J. W. Palmer & A. J. Holloway,
TWODPSEC, Some Additions to Figaro,
Starlink User Note 16.5, December 1997.